The object of the present invention is directed to an improvement in the running conditions of plants operated according to an absorption tri-thermal cycle. Such plants operate by delivering heat to an external medium in a temperature range A, by recovering heat from an external medium in a temperature range B, and by exchanging heat with an external medium in a temperature range C. Two cases are to be considered.
In the first case, hereinafter called case 1, the temperature range B is at least partly higher than the temperature range A and the device receives heat in the temperature range C, which is at least partly higher than the temperature ranges A and B: this device may operate either as refrigerator when the temperature range C is below room temperature, or as a heat pump when the temperature range C is at room temperature or above. In the first case a heat amount Q.sub.2 is delivered in the temperature range A, which amount is greater than the heat amount Q.sub.1 received in the temperature range B, while a heat amount Q.sub.2 -Q.sub.1 is received in the temperature range C. In the second case, hereinafter called case 2, the temperature range B is at least partly below the temperature range A, and the system delivers heat in the temperature range C which is at least partly below the temperature ranges A and B: this is the case of the heat converter which is the object of the French Pat. No. 2,321,098. In case 2, a heat amount Q.sub.2 is delivered in the temperature range A, which amount is smaller than the heat amount Q.sub.1 received in the temperature range B, while a heat amount Q.sub.2 -Q.sub.1 is released in the temperature range C.
In the following description, the term "absorption heat pump" will refer to any device receiving heat only above room temperature and complying with the above general definition, either in case 1 or in case 2.
In both cases 1 and 2, the cycle comprises at least one absorption step in which a gas phase of a working fluid, acting as solute, is contacted with a liquid phase used as solvent, and a desorption step in which the solution obtained in the absorption step is converted again to a liquid phase of low solute content and a gas phase of high solute content.
The desorption step is usually effected by feeding the solution S obtained in the absorption step into an enclosure heated by indirect contact exchange with an external fluid, while recovering a liquid phase L of low solute content and a vapor phase V of high solute content according to the device shown in FIG. 1a. When the solvent phase is itself volatile, it is usual in the absorption refrigerators to rectify the vapor phase V by condensing it at least partly, feeding back a part of the condensed liquid phase as reflux, and counter-currently contacting the vapor phase V with the reflux, for example in a plate column.
The stripping of the solvent phase by vapor V may be reduced by operating according to the arrangement represented in FIG. 1b. The solution S is supplied to the top of a counter-current contact zone and a vapor phase V' is generated at the bottom of this contact zone by vaporizing a part of the liquid phase L''' discharged from the bottom of said zone. The latter comprises, for example, plates, such as those commonly used for distillation or absorption. When contacting the solution S in that counter-current zone, the vapor increases its solute content and reduces its solvent content, while the solution decreases its solute content and increases its solvent content. The liquid phase L of decreased solute content is discharged from the vaporizer W. This problem is made clearly apparent when considering the desorption of an ammonia solution.
When operating according to the embodiment shown in FIG. 1a, at a pressure of 8 kg/cm.sup.2 with a solution containing 50% b.w. of NH.sub.3, the following evolution of the concentrations of liquid phase L and vapor phase V versus temperature is observed.
______________________________________ Temperature .degree.C. 54.2 71.3 85.7 102.8 142.4 NH.sub.3 fraction in liquid phase b.w. 0.5 0.4 0.33 0.25 0.09 NH.sub.3 fraction in vapor phase b.w. 0.99 0.97 0.94 0.88 0.5 ______________________________________
It is found that a temperature increase results in a liquid phase containing less and less ammonia, but the amount of water stripped in vapor phase quickly increases. At 142.4.degree. C., the solution vaporizes entirely.
Conversely, when operating according to the arrangement shown in FIG. 1b, it is possible to maintain the vapor V evolved from the counter-current contact zone at a concentration close to the balance with the initial solution, i.e. 99% ammonia in fraction b.w. for a pressure of 8 kg/cm.sup.2 and a solution containing 50% b.w. of NH.sub.3, while obtaining a liquid phase L whose ammonia concentration corresponds to the heating temperature in the bottom of the counter-current zone, which provides for a thorough desorption. Such an arrangement is disclosed particularly in the above-mentioned French Patent.
However this arrangement requires the supply of the whole heat at a temperature close to the saturation temperature of the liquid phase L discharged from the counter-current contact zone, which temperature is the higher as the desorption is the more complete.